1. Technical Field
The present invention discloses a composition of a stable suspension of a poorly water soluble compound comprising particles of the compound suspended in a frozen aqueous matrix and method for its preparation. The composition is stable for a prolonged period of time, preferably six months or longer.
2. Background Art
There is an ever increasing number of pharmaceutical compounds being formulated that are poorly soluble or insoluble in aqueous solutions. Such compounds provide challenges to delivering them in an injectable form. Drugs that are insoluble in water can have significant benefits when formulated as a stable suspension of sub-micron particles. Accurate control of particle size is essential for safe and efficacious use of these formulations. Particles must be less than seven microns in diameter to safely pass through capillaries without causing emboli (Allen et al., 1987; Davis and Taube, 1978; Schroeder et al., 1978; Yokel et al., 1981). One solution to this problem is the production of extremely small particles of the insoluble drug candidate and the creation of a microparticulate or nanoparticulate suspension. In this way, drugs that were previously unable to be formulated in an aqueous based system can be made suitable for intravenous administration. Suitability for intravenous administration includes small particle size (<7 μm), low toxicity (as from toxic formulation components or residual solvents), and bioavailability of the drug particles after administration.
Suspensions may also be suitable for oral, intramucscular, pulmonary, topical or subcutaneous administration. When administered by these routes, it may be desirable to have particle size in the range of 5 to 100 microns.
Suspensions may lack sufficient physical and chemical stability when stored for a prolonged period of time. Physical instability occurs when the particles aggregate to form larger particles, which is generally the result of small particle size. Ostwald-Mie ripening may occur due to the small particle radius and attendant increase in surface activity, hence solubility. In particular, nanoparticles have a very high surface-to-volume ratio which enhances their dissolution rate and solubility. As a result, the particles may solubilize in the suspension followed by recrystallization to form large crystals. Aggregation and crystal growth result in suspensions of nanoparticles with larger and varying particle sizes. Suspensions with particles larger than 7 μm are no longer suitable for intravenous administration.
In a suspension, the active ingredient may also undergo degradation and result in reduced activity over time due to interaction with the suspension medium. Even slight dissolution may accelerate the degradation of the active ingredient. The rate of chemical degradation depends on particle size, intrinsic solubility, and the chemical nature of the active ingredient.
It is highly desirable to have a pharmaceutical preparation of an aqueous suspension with a long shelf life, preferably a minimum of six months in terms of both physical and chemical stabilities.
Several methods have been described in the prior art to limit aggregation and crystal growth of nanoparticles in suspension to improve their physical stability and shelf-life. One method includes the step of adding surface stabilizers to the preparations. Suitable surface stabilizers include surfactants, polymers, cloud point modifiers (see U.S. Pat. Nos. 5,298,262; 5,346,702; and 5,470,583), crystal growth modifiers (see U.S. Pat. No. 5,665,331), and cryoprotectants (see U.S. Pat. No. 5,302,401). While such approaches have found success in limiting-particle aggregation and crystal growth, suitable surface-active agents may not be found that would enable extended storage of the suspension in the liquid state, either at room temperature or in the refrigerator. Or, if stabilizing agents could be found, they may possess undesirable toxicity profiles.
Another approach to inhibiting the aggregation and crystal growth of nanoparticles is to limit the average particle size to a narrow range of from about 150 nm to about 350 nm, as described by Liversidge et al. in U.S. Pat. No. 6,267,989. The '989 patent discloses that aggregation and crystal growth are minimized when the particles are within this size range. However, the narrow range of the particle sizes limits its applications. For certain applications, it may be desirable to have nanoparticle suspensions with particle sizes in excess of 400 nm. These applications include, but are not limited to, oral, subcutaneous, or intramuscular administration in which the desirable particle size may be from 5 to 100 microns. In other formulations, the desirable particle size may be smaller than 100 nm. This is true, for example, for particles designed to evade the RES (reticuloendothelial system). Such long-circulating particles can also migrate across loose, fenestrated vasculature such as that associated with certain cancerous tumors. This would facilitate passive targeting of such tumors.
Yiv et al. discloses in U.S. Pat. No. 6,245,349 a stable formulation of lipid nanoparticles of lipophilic and amphipathic drugs. The formulation is an oil-in-water microemulsion consisting of phospholipid, propylene glycol, polyethylene glycol, a surfactant and water. An oil component such as a triglyceride is optional. The components are blended together to form an emulsion. The average particle size should be smaller than 200 nm-for the preparation to be filter sterilized. The composition can be stored either in a concentrated form or a diluted form. The diluted form includes an aqueous buffer and is stable at a temperature range of about −50° C. to about 40° C. In Example 1, the composition was stored at −20° C. for 21 days with no evidence of phase separation, change in particle size, or drug crystallization. The method, however, is limited to oil-in-water dispersions with particle sizes smaller than 200 nm, wherein all components are liquids. Such dispersions are commonly sterilized by filter sterilization which requires the dispersion be passed through filters with a pore size of 220 nm.
The prior art also describes methods of improving the chemical stability of nanoparticle preparations for prolonged storage. The general approach is to remove the aqueous medium by lyophilization and store the nanoparticles in dry, lyophilized form. An example is disclosed in Example 6 of U.S. Pat. No. 5,091,187. Dialysis is generally required before lyophilization to remove any unwanted solutes, such as salt, or to prevent the concentration of such solutes during the lyophilization process. The additional steps of dialysis and lyophilization increase production costs since dialysis is a very time consuming process and lyophilization is an energy consuming process. Furthermore, the lyophilized preparation requires reconstitution with an appropriate dispersing medium before administration either by injection (intravenously, intramuscularly, or subcutaneously), or orally. Such requires more labor in administering the pharmaceutical agent as well as introducing potential human errors that can occur during reconstitution.
As part of an effort to develop new methods for stabilization of these suspensions, we have discovered that freezing may circumvent these instability mechanisms by encasing the drug particles in a frozen aqueous matrix. At such low temperatures, drug solubility is reduced and very high viscosity of the aqueous medium disfavors diffusion of solute drug away from the solid particle. This includes nucleation, crystal growth and Ostwald ripening. Lower temperatures also increase chemical stability by slowing down drug degradation in the aqueous medium. Crystallization of water may also occur, for example below the eutectic point of the mixture, thus eliminating the possibility of forming a solution phase containing drug which can undergo secondary nucleation, crystal growth and Ostwald ripening.
The nanoparticles in the invention can be prepared from any of the known methods in the art. One approach centers on reducing the size of the particles that deliver the drug. In one such series of patents, which include U.S. Pat. Nos. 6,228,399; 6,086,376; 5,922,355; and 5,660,858, Parikh et al. discloses that sonication may be used to prepare microparticles of the water-insoluble compound. Of these patents, U.S. Pat. No. 5,922,355 discloses an improvement to a method that uses sonication for making smaller particles. The improvement comprises mixing an active pharmacological agent with a phospholipid and surfactants in a single-phase aqueous system and applying energy to the system to produce smaller particles. Stabilization of the suspension by freezing is not disclosed, however.
U.S. Pat. No. 5,091,188, issued to Haynes, also discloses reducing the size of particles of a pharmacologically active water-insoluble drug and employing a lipid coating on the particles to confer a solid form. The patent is directed to a pharmaceutical composition consisting essentially of an aqueous suspension of solid particles of the drug having a diameter of about 0.05 to about 10 microns. The lipid coating affixed to the surface of the particles acts to stabilize them. The composition is produced by adding the drug to water in the presence of membrane-forming lipid surfactants and then reducing the particle size within the aqueous suspension. However, freezing the suspension is not disclosed as a stabilization method.
U.S. Pat. No. 5,858,410 discloses a pharmaceutical nanosuspension suitable for parenteral administration. The '410 patent discloses subjecting at least one solid therapeutically active compound dispersed in a solvent to high pressure homogenization in a piston-gap homogenizer to form particles having an average diameter, determined by photon correlation spectroscopy (PCS) of 10 nm to 1000 nm, the proportion of particles larger than 5 microns in the total population being less than 0.1% (number distribution determined with a Coulter counter), without prior conversion into a melt, wherein the active compound is solid at room temperature and is insoluble, only sparingly soluble or moderately soluble in water, aqueous media and/or organic solvents. The Examples in the '410 patent disclose jet milling prior to homogenization.
U.S. Pat. No. 5,145,684 discloses another approach to providing nanoparticles of insoluble drugs for parenteral delivery by reducing the size of the particles. The '684 patent discloses the wet milling of an insoluble drug in the presence of a surface modifier to provide a drug particle having an average effective particle size of less than 400 nm. The '684 patent emphasizes the desirability of not using any solvents in its process. The '684 patent discloses the surface modifier is adsorbed on the surface of the drug particle in an amount sufficient to prevent agglomeration into larger particles.
Besides physically reducing the size of drug particles and coating the particles with a surface stabilizer, nanoparticles can also be prepared by the various methods of precipitation. These methods typically involve dissolving the drug in a solvent as a continuous phase followed by changing the conditions of the solution to a non-continuous phase so that fine particles of the drug precipitate out into the non-continuous phase. A coating agent or surface stabilizer is normally used to co-precipitate with the drug to stabilize the particles. Examples of these precipitation methods are solvent and anti-solvent microprecipitation, phase inversion precipitation, pH shift precipitation, supercritical fluid precipitation, and temperature shift precipitation.
Examples of appropriate precipitation techniques include preparing nanoparticle suspensions as disclosed in U.S. Patent Application Ser. Nos. 60/258,160; 09/874,799; 09/874,637; 09/874,499; and 09/953,979, which are incorporated herein by reference and made a part hereof. These applications disclose forming small particles of organic compounds by dissolving the organic compound in a water miscible organic solvent followed by precipitating the organic compounds in an aqueous medium to form a pre-suspension followed by adding energy to the pre-suspension to stabilize a coating of the particle, to alter the lattice structure of the particle or to reduce particle size. The process is preferably used to prepare a suspension of a poorly water-soluble, pharmaceutically active compound.
U.S. Pat. No. 5,118,528 discloses a process for preparing nanoparticles by solvent anti-solvent precipitation. The process includes the steps of: (1) preparing a liquid phase of a substance in a solvent or a mixture of solvents to which may be added one or more surfactants, (2) preparing a second liquid phase of a non-solvent or a mixture of non-solvents, the non-solvent is miscible with the solvent or mixture of solvents for the substance, (3) adding together the solutions of (1) and (2) with stirring; and (4) removing of unwanted solvents to produce a colloidal suspension of nanoparticles. The '528 patent discloses that it produces particles of the substance smaller than 500 nm without the supply of energy. In particular, the '528 patent states that it is undesirable to use high energy equipment such as sonicators and homogenizers.
U.S. Pat. No. 4,826,689 discloses a method for making uniformly sized particles from water-insoluble drugs or other organic compounds. First, a suitable solid organic compound is dissolved in an organic solvent, and the solution can be diluted with a non-solvent. Then, an aqueous precipitating liquid is infused, precipitating non-aggregated particles with substantially uniform mean diameter. The particles are then separated from the organic solvent. Depending on the organic compound and the desired particle size, the parameters of temperature, ratio of non-solvent to organic solvent, infusion rate, stir rate, and volume can be varied according to the invention. The '689 patent discloses this process forms a drug in a metastable state which is thermodynamically unstable. The '689 patent discloses trapping the drug in a metastable state by utilizing crystallization inhibitors (e.g., polyvinylpyrrolidinone) and surface-active agents (e.g., poly(oxyethylene)-co-(oxypropylene)) to render the metastable precipitate stable enough to be isolated by centrifugation, membrane filtration or reverse osmosis.
U.S. Pat. No. 5,780,062 discloses a method of preparing small particles of insoluble drugs by (1) dissolving the drug in a water-miscible first solvent, (2) preparing a second solution of a polymer and an amphiphile in an aqueous second solvent in which the drug is substantially insoluble whereby a polymer/amphiphile complex is formed and (3) mixing the solutions from the first and second steps to precipitate an aggregate of the drug and polymer/amphiphile complex.
U.S. Pat. No. 4,997,454 discloses a method for making uniformly sized particles from solid compounds. The method of the '454 patent includes the steps of dissolving the solid compound in a suitable solvent followed by infusing precipitating liquid thereby precipitating non-aggregated particles with substantially uniform mean diameter. The particles are then separated from the solvent. The '454 patent discourages forming particles in a crystalline state because during the precipitating procedure the crystal can dissolve and recrystallize thereby broadening the particle size distribution range. The '454 patent encourages during the precipitating procedure to trap the particles in a thermodynamically unstable particle state.
U.S. Pat. Nos. 6,235,224 B1 and 6,143,211, both issued to Mathiowitz et al., disclose the use of phase inversion phenomena to precipitate microencapsulated microparticles. The method includes mixing a polymer and a drug with a solvent. This mixture is introduced into an effective amount of a miscible non-solvent, thereby causing spontaneous formation of the microencapsulated product.
Microprecipitation by pH shifting is another technology used to prepare dispersions of a nanoparticulate pharmaceutical agent. See, e.g., U.S. Pat. Nos. 5,766,635; 5,716,642; 5,665,331; 5,662,883; 5,560,932; and 4,608,278. This technology involves dissolving a pharmaceutical compound in an aqueous base having a non-neutral pH that is then neutralized to precipitate the compound in the aqueous base.
In yet another approach, such as that disclosed in U.S. Pat. No. 5,766,635, issued to Spenlenhauer et al., nanoparticles have been prepared by dissolving a poly(ethylene) oxide and/or poly(propylene) oxide/polylactide copolymer in an organic solvent, mixing the organic solution so formed with an aqueous solution to cause nanoparticles to precipitate out of solution, and microfluidizing the suspension without the use of surfactants. Carrier particles consisting of a solid polymer matrix are thus formed, into which a co-precipitated pharmaceutical agent may be incorporated.
Precipitation by supercritical fluid is disclosed by U.S. Pat. Nos. 5,360,478 and 5,389,263 to Krukonis et al., and WO 97/14407 to Johnston. The technology is similar to the solvent anti-solvent precipitation method. In this case, the supercritical fluid, which can be a gas or liquid at conditions of pressure and temperature above its critical point, acts as the anti-solvent. The addition of the supercritical fluid to a solution of a solute in a solvent causes the solute to attain or approach supersaturated state and to precipitate out as fine particles.
Temperature shift precipitation is disclosed in U.S. Pat. No. 5,188,837 to Domb. The method involves adding a thermally stable drug to a polymer. The polymer is often oil-based (e.g., phospholipid, synthetic waxes) and has a low melting point. The drug is heated with the polymer to slightly above the melting point of the polymer to form a warm emulsion of the drug in the molten polymer. The emulsion is then cooled quickly by adding the emulsion to a bath of cold non-solvent, such as water, with vigorous shaking to cause the emulsion to form droplets and to solidify to entrap the active agent in a suspension.
Yet another approach to preparing submicron particles of poorly water soluble organic compounds is the formation of an emulsion of the compound. The organic compound is dissolved in an organic phase. The organic phase forms an emulsion with an aqueous phase. An emulsion evaporation method is disclosed in U.S. patent application Ser. No. 09/964,273. The method includes the steps of: (1) providing a multiphase system having an organic phase and an aqueous phase, the organic phase having a pharmaceutically effective compound therein; and (2) sonicating the system to evaporate a portion of the organic phase to cause precipitation of the compound in the aqueous phase and having an average effective particle size of less than about 400 nm.
U.S. Pat. No. 5,605,785 discloses a process for forming nanoamorphous dispersions of photographically useful compounds. The process of forming nanoamorphous dispersions include any known process of emulsification that produces a dispersed phase having amorphous particulates.
Still yet another approach to preparing submicron size nanoparticle suspension of a pharmaceutically active compound is by seeding at some point during a precipitation process to generate crystals of a desired morphology. (see U.S. patent application Ser. No. 10/035,821). The method comprises the steps of dissolving a first quantity of the pharmaceutically-active compound in the water-miscible first organic solvent to form a first solution. The first solution is then seeded. Alternatively, a second solvent may be seeded. It is also possible to use seed compounds at other points during the precipitation process. The first solution is then mixed with the second solvent. The mixing of the first solution with the second solvent results in the precipitation of the pharmaceutically-active compound in a desired morphological form.
Another approach is directed to the production of suspended particles coated with protein. U.S. Pat. No. 5,916,596, issued to Desai et al., discloses the application of high shear to a mixture of an organic phase having a pharmacologically active agent dispersed therein and an aqueous medium containing a biocompatible polymer. The mixture is sheared in a high-pressure homogenizer at a pressure in the range of from about 3,000 to 30,000 psi. The '596 patent requires the mixture contain substantially no surfactants because the combined use of a surfactant with a protein results in the formation of large, needle-like crystalline particles that increase in size during storage. See columns 17–18, example 4.
U.S. Pat. No. 5,560,933, issued to Soon-Shiong et al., discloses the formation of a polymeric shell around the water-insoluble drug for in vivo delivery. The method discloses the application of sonication to a mixture comprising a polymer-containing aqueous medium and a dispersing agent having a substantially water-insoluble drug dispersed therein. In this reference, sonication is used to drive the formation of disulfide bonds in the polymer, causing it to cross-link so as to produce a polymeric shell around the drug. Sonication is conducted for a time sufficient for the disulfide bonds to form.
In U.S. Pat. No. 5,665,383, Grinstaff et al. discloses the application of ultrasound to a single-phase B i.e., an aqueous medium—to encapsulate an immunostimulating agent within a polymeric shell for in vivo delivery. The ultrasound promotes crosslinking of the encapsulating agent by disulfide bonds to form the shell.
U.S. Pat. Nos. 5,981,719 and 6,268,053 disclose a method of preparing microparticles of macromolecules with particle size of less than 10 microns. Macromolecules are mixed with a soluble polymer or mixture of soluble polymers (e.g., albumin) at a pH near the isoelectric point of the macromolecule in the presence of an energy, preferably heat, for a predetermined length of time. The microparticles formed by this process allow aqueous fluids to enter and solubilized macromolecules and polymers to exit the microparticles and can be made to exhibit short-term or long-term release kinetics, thereby providing either rapid or sustained release of macromolecules.